In quiz you can give a maximum grade with decimals. If you do that you cannot change the format or increase decimal places it has in grade book. With the whole numbers you can.
- Create a quiz
- Set the maximum grade for any number with decimals except .00 (e.g. 5.5)
- Submit a quiz attempt
- Go to grade book and select 'Setup' and edit (-> edit settings) the quiz activity
- change the Grade display type or increase the amount of overall decimal places
- Save --> Nothing happens...
- If you go back to the quiz and change the maximum grade to whole number e.g. 10 and repeat 1-6 (save and go back to the grade book to edit the grade display type or increase the amount of decimals) that actually saves the setup and you will see the grades with the format you wanted.